1. Technical Field
The present invention relates to a battery charging apparatus for rechargeable batteries used for portable computers, model machinery, headphone stereos, and portable cordless telephones, and more particularly to a mode-convertible charging apparatus operational under a constant voltage and a constant current charging mode.
2. Related Art
Generally speaking, second batteries mean batteries which can be reused and recharged by a power supply through a reversible reaction after use. Recently, first battery charging apparatuses have been commercialized. However, this starts from the possibility that first batteries can also be charged rather than the characteristics of the charging apparatuses. Therefore, first batteries can not be recharged as many times as second batteries.
Rechargeable batteries are more economical in case batteries have to be replaced frequently. Also, due to a desirable discharging characteristic they are unavoidable for the model machinery and portable machinery which require high current. Recently, various kinds of batteries have been commercialized, ranging from 500 mAh to several Ah capacity, but the charging apparatuses are not yet as diversified as the batteries.
Compared to regular batteries, rechargeable batteries have very low internal resistance, and therefore can provide a large amount of current instantaneously, and also can keep a stable voltage until a discharge terminal voltage is reached because of a good discharge characteristic. Here, a discharge terminal voltage means a limit voltage of a terminating discharge in battery tests. To charge rechargeable batteries, a proper amount of current should be introduced into the batteries. Charging period can be reduced with a high current, but there is a risk of overcharge (charging beyond a full charge limit). In case of overcharge, most batteries produce gas internally which can be absorbed in case the amount of gas is small. A large amount of gas can be produced under high charging current however, and can rupture overcharged rechargeable batteries.
Lead batteries which are popular in cars and in emergency power supplies are cheap, but they are not adequately protected from overcharge or overdischarge. Nickel Cadmium (Ni--Cd) batteries have superior characteristics in charge-discharge and maintenance. However, they are losing popularity because of a low volume energy density (Wh/l) and, especially, due to environmental contamination (heavy metal contamination such as Cadmium). In the design of an automatic battery charging apparatus, the most important task is the determination of the charge completion point (the point where the battery is fully charged). Electrical detection of the increase of terminal voltage of a battery is the only way to accomplish the latter task.
Exemplar recent efforts in the art include U.S. Pat. No. 5,656,917 to Theobald, entitled Battery Identification Apparatus And Associated Method, U.S. Pat. No. 5,648,715 to Patino et al., entitled Method And Apparatus For Current Compensation Of A Battery In A Charger, U.S. Pat. No. 5,644,210 to Hwang, entitled Charging Control Method And Circuit Of Recharging Battery, U.S. Pat. No. 5,637,981 to Nagai et al., entitled Method For Charging A Secondary Battery And Charger Used Therefor Using Constant Current And Constant Voltage, U.S. Pat. No. 5,631,537 to Armstrong, entitled Battery Charge Management/Protection Apparatus, U.S. Pat. No. 5,612,607 to Nicolai, entitled Method For The Fast Charging Of A Battery And Integrated Circuit For The Implementation Of This Method, U.S. Pat. No. 5,541,496 to Simmonds, entitled Apparatus And Method Of Rapidly Charging Nickel-Cadmium Batteries, U.S. Pat. No. 5,541,490 to Sengupta et al., entitled Computer Power Supply System, U.S. Pat. No. 5,510,690 to Tanaka et al., entitled Battery Pack, Battery Discrimination Control Apparatus And Method Therefor, U.S. Pat. No. 5,504,416 to Hooloway et al., entitled Battery Charger Circuit Including Battery Temperature Control, U.S. Pat. No. 5,500,584 to Shimomoto, entitled Battery Charging Method And Apparatus Using Initial Charging Step With Gradually Increasing Charging Current, Quick Charging Step With Large Charging Current And Final Charging Step With Decreasing Charging Current, U.S. Pat. No. 5,489,834 to Pitkanen, entitled Battery Type And Temperature Identification Circuit, U.S. Pat. No. 5,485,090 to Stephens, entitled Method And Apparatus For Differentiating Battery Types, U.S. Pat. No. 5,485,073 to Kasashima et al., entitled Personal Computer For Performing Charge And Switching Control Of Different Types Of Battery Packs, U.S. Pat. No. 5,438,248 to Hyuck, entitled Method And Apparatus For Recognizing Different Types Of Batteries, U.S. Pat. No. 5,345,392 to Mito et al., entitled Battery Charge Monitor For A Personal Computer, U.S. Pat. No. 5,321,627 to Reher, entitled Battery Monitor And Method For Providing Operating Parameters, U.S. Pat. No. 5,200,690 to Uchida, entitled Quick Charge Control Apparatus And Control Method Thereof, U.S. Pat. No. 5,193,067 to Sato et al., entitled Battery Condition Detection Apparatus, U.S. Pat. No. 5,027,294 to Fakruddin et al., entitled Method And Apparatus For Battery-Power Management Using Load-Compensation Monitoring Of Battery Discharge, U.S. Pat. No. 4,707,795 to Alber et al., entitled Battery Testing And Monitoring System, and U.S. Pat. No. 4,377,787 to Kikuoka et al., entitled System For Measuring State Of Charge Of Storage Battery.
As evidenced by the foregoing efforts in the art, the change of terminal voltage in charging depends on individual battery state or aging change, and a perfect automatic charging apparatus is highly difficult to achieve. Compromise has to be made among the protection, fast and sufficient charging of battery (in case of high capacity rechargeable batteries using an electrolyte, measurement of a density of the electrolyte is the best way of determining the charging completion point).
There are two ways to charge batteries. One is a constant voltage charging method and the other is a constant current charging method. In the constant voltage charging method, batteries are charged under a constant voltage higher than the nominal voltage in a certain ratio. This method is widely used in an emergency power supply charging mode where a complete charge or discharge are not common. This mode is also called an alternating-current floating charge mode because batteries are charged in a normal period, but they are discharged in case a load is higher than the charging power supply. This method is advantageous in that extra timing apparatuses for battery protection are not necessary, but it is disadvantageous because a large amount of current in the initial charging period can hurt battery or power supply. To overcome this problem, current controlled resistance could be used to control initial current. An increase in voltage however, during the charging period reduces charging current. Therefore, the charging period also increases and sufficient charging is difficult to achieve. This method is popular for economical reasons. In the telephone handset charging apparatus, current is supplied for about twenty hours charging, and charging voltage is kept at higher than the defined value for the alternating-current floating voltage so as to control initial current, incomplete charging and reduction of charging period. This method is common in inexpensive products. The popularity of this method in a telephone handset starts from the high tolerance of nickel cadmium battery against overcharge, in addition to a simple circuit structure composed of a power supply and a resistance.
The constant current charging method charges a battery with a constant current regardless of the increase in battery terminal voltage during the charge process, as explained below. This mode is employed in initial charging and fast charging. In this method, timed charging should be used unless overcharge causes a shortening of battery life. Also, a constant current power supply is necessary in this method because constant current should be supplied to a battery in spite of the increase in battery terminal voltage during a charging process. According to this method, batteries are charged by a constant current, so that the charging period can be reduced and sufficient charging is possible, but overcharge can cause a fatal damage on a battery compared to an overcharge in the constant voltage charging mode. For rapid charging in a constant current charging mode, the power supply should be disconnected at the charging completion point, which is not easy to determine based on the charging period unless a battery is charged after a complete discharge. This means that a 50% discharged battery requires less time than a completely discharged battery in fast charging.
In some of the commercialized fast charging apparatus (e.g., those used in walkman stereo headphones), batteries are not charged immediately, but are discharged for a certain period which examining the drop of terminal voltage and determining the fast charging period (based on a pre-programmed procedure) when they are placed for charging. Again, in this method, the same level of charging is difficult to achieve unless a prediction on an aging change is included; in addition, this method cannot be applied to different model batteries in the same way.
Therefore, there is a need in the prior art for development of a mode-convertible battery charging apparatus which can precisely determine a completely charged state of a rechargeable battery, and which is free from damage resulting from static, surge or noise on removal of the battery. There is also need in the prior art for development of a battery charging apparatus which can detect the type of rechargeable battery pack, and can selectively perform the best charging method (constant current or constant voltage charging) based on the type of battery pack.